A key requirement on Long Term Evolution (LTE) currently being standardized in 3GPP is frequency flexibility, and for this purpose carrier bandwidths between 1.4 MHz and 20 MHz are supported, as is both frequency division duplex (FDD) and time division duplex (TDD), so that both paired and unpaired spectrum can be used. For FDD the downlink (DL), i.e. the link from the base stations to the mobile terminals, and uplink (UL), i.e. the link from the mobile terminals to the base stations, use different frequencies and can hence transmit simultaneous. For TDD, uplink and downlink use the same frequency and cannot transmit simultaneously. Uplink and downlink can however share the time in a flexible way, and by allocating different amounts of time, such as the number of subframes of a radio frame, to uplink and downlink, it is possible to adapt to asymmetric traffic and resource needs in uplink and downlink.
The above asymmetry also leads a significant difference between FDD and TDD. Whereas for FDD the same number of uplink and downlink subframes is available during a radio frame, for TDD that the number of uplink and downlink subframes may be different. One of many consequences of this is that in FDD a mobile terminal can always send feedback in response to a DL assignment of resources in an UL subframe subject to a certain fixed processing delay. In other words, every DL subframe can be associated to a specific later UL subframe for feedback generation in way that this association is one-to-one, i.e. to each UL subframes is associated exactly one DL subframe. For TDD however, since the number of UL and DL subframes during a radio frame may be different, it is in general not possible to construct such one-to-one association. For the typical case with more DL subframes than UL subframes, it is rather so that feedback from several DL subframes needs to be transmitted in each UL subframe.
In LTE, a radio frame of 10 ms duration is divided into ten subframes, each 1 ms long. In case of TDD, a subframe can be assigned to uplink or downlink, i.e. uplink and downlink transmission cannot occur at the same time. Furthermore, each 10 ms radio frame is divided into two half-frames of 5 ms duration where each half-frame comprises of five subframes.
The first subframe of a radio frame is always allocated to DL transmission. The second subframe is a special subframe and it is split into three special fields, DwPTS, GP and UpPTS, with a total duration of 1 ms. UpPTS is used for uplink transmission of sounding reference signals and, if so configured, reception of a shorter random access preamble. No data or control signaling can be transmitted in UpPTS. GP is used to create a guard period between periods of DL and UL subframes and may be configured to have different lengths in order to avoid interference between UL and DL and is typically chosen based on the supported cell radius. The DwPTS field is used for downlink transmission much like any other DL subframe with the difference that it has shorter duration.
Different allocations of the remaining subframes to UL and DL are supported, both allocations with 5 ms periodicity in which the first and second half-frame have identical structure, and allocations with 10 ms periodicity for which the half-frames are organized differently. For certain configurations the entire second half-frame is assigned to DL transmission.
In the DL of LTE, Orthogonal Frequency Division Multiplex (OFDM) with a sub carrier spacing of 15 kHz is used. In the frequency dimension the subcarriers are grouped into resource blocks, each containing twelve consecutive subcarriers. The number of resource blocks depends on the system bandwidth, and the minimum bandwidth corresponds to six resource blocks. Depending on the configured cyclic prefix length a 1 ms subframe contains either 12 or 14 OFDM symbols in time. The term resource block is also used to refer to the two-dimensional structure of all OFDM symbols within a subframe, times a resource block of subcarriers. The downlink part of the special subframe DwPTS has a variable duration, and can assume lengths of 3, 9, 10, 11 or 12 OFDM symbols for the case with normal cyclic prefix, and 3, 8, 9 or 10 symbols for the case with extended cyclic prefix.
In order to improve performance of transmission in both the DL and UL direction, LTE uses Hybrid-ARQ (HARQ). The basic idea of HARQ is that after receiving data in a DL subframe the terminal attempts to decode it and then reports to the base station whether the decoding was successful (ACK, acknowledgement) or not (NAK, negative acknowledgement). In case of an unsuccessful decoding attempt the base station thus receives a NAK in a later UL subframe, and can retransmit the erroneously received data. Downlink transmissions may be dynamically scheduled, i.e. in each subframe the base station transmits control information on which terminals are to receive data and upon which resources in the current DL subframe. By resources is here meant some set of resource blocks. The control signaling is transmitted in the first 1, 2 or 3 OFDM symbols in each subframe. (For system bandwidth < or =10, it is transmitted in the first 2, 3 or 4 OFDM symbols in each subframe). The data sent to a terminal in a single DL subframe is often referred to a transport block.
A terminal will thus listen to the control channel, and if it detects a DL assignment addressed to itself it will try to decode the data. It will also generate feedback in response to the transmission, in the form of an ACK or a NAK depending on whether the data was decoded correctly or not. Furthermore, from the control channel resources on which the assignment was transmitted by the base station, the terminal can determine the corresponding Physical Uplink Control Channel resource (PUCCH) in case that the ACK/NACK is transmitted on the PUCCH. The PUCCH resource may also be configured by the network, which is the case when a channel quality report or a scheduling request is transmitted at the same time as the ACK/NAK feedback is to be provided.
For LTE FDD the terminal will in response to a detected DL assignment in subframe n, send an ACK/NAK report in uplink subframe n+4. For the case with so-called Multiple In Multiple Out (MIMO) multi-layer transmission two transport blocks are transmitted in a single DL subframe, and the terminal will respond with two ACK/NAK reports in the corresponding uplink subframe.
The assignment of resources to the terminals is handled by the scheduler, which takes into account traffic and radio conditions so as to use the resources efficiently while also meeting delay and rate requirements. Scheduling and control signaling may be done on a subframe to subframe basis, i.e. each downlink subframe is scheduled independently of others.
As described above, the first step for a terminal to receive data from the base station in a DL subframe is to detect a DL assignment in the control field of a DL subframe. In the case that the base station sends such an assignment but the terminal fails to decode it, the terminal does obviously not know that is was scheduled and will hence not respond with an ACK/NAK in the uplink. This situation is referred to as a missed DL assignment. If the absence of an ACK/NAK can be detected by the base station, it can take this into account for subsequent retransmissions. Typically the base station should at least retransmit the missing packet, but it may also adjust some other transmission parameters.
There is not a one-to-one relation between UL and DL subframes as discussed above. Thus the terminal cannot always send an ACK/NAK in response to a DL assignment in subframe n in UL subframe n+4, since this subframe may not be allocated to UL transmission. Hence each DL subframe may be associated with a certain UL subframe subject to a minimum processing delay, meaning that ACK/NAKs in response to DL assignments in subframe n are reported in subframe n+k with k>3. Furthermore, if the number of DL subframes is larger than the number of UL subframes, ACK/NAKs in response to assignments in multiple DL subframes may need to be sent in a single UL subframe. For a given UL subframe, the number of associated DL subframes depends on the allocation of subframes to UL and DL, and can be different for different UL subframes within a radio frame.
Since DL assignments can be given independently across DL subframes, a terminal may be assigned DL transmissions in multiple DL subframes that are all to be acknowledged in a single UL subframe. Hence the uplink control signaling needs to support, in some way, feedback of ACK/NAKs from multiple DL transmissions from a terminal in a given UL subframe.
One obvious way to approach the above problem is to allow the terminal to transmit multiple individual (for each DL transmission) ACK/NAK bits in a single UL subframe. Such protocols have however worse coverage than transmission of one or two ACK/NAK reports. In addition, the more ACK/NAKs that are allowed to be transmitted from a single terminal, the more control channel resources need to be reserved in the uplink. To improve control signaling coverage and capacity, it has been agreed to do some form of compression, or bundling, of ACK/NAKs. This means that all ACK/NAKs that are to be sent in a given UL subframe are combined into a smaller number of bits, such as a single ACK/NAK report. As an example, the terminal can transmit an ACK only if the transport blocks of all the DL subframes were received correctly and hence to be acknowledged. In any other case, meaning that a NAK for at least one DL subframe is to be transmitted, a combined NAK is sent for all DL subframes.
Hence, as described above, to each UL subframe in TDD a set of DL subframes can be associated, rather than a single subframe as in FDD, for which DL transmissions are to be given ACK/NAK response in the given UL subframe. In the context of bundling this set is often referred to as the bundling window. The two basic approaches then include:                Multiplexing of multiple ACK/NAKs, meaning that multiple individual ACK/NAK reports of the subframes are fed back. For the case with no MIMO, and a configuration with 3 DL subframes (including DwPTS) and two UL subframes as depicted in FIG. 2b, up to two bits of ACK/NAK feedback is fed back in subframe #2 and #7 and one up to one bit in subframes 3 and 8. In the general case, there may be a third state so that ACK/NACK/DTX is fed back. DTX then represents that the terminal did not receive/detect any assignment during in the corresponding DL subframe.        Bundling of multiple ACK/NAKs, meaning that a single ACK/NAK is generated from the individual ACK/NAKs and that this single ACK/NAK is fed back. For the case with no MIMO, the terminal combines the ACK/NAKs of multiple DL subframes so that a single ACK/NAK is generated and fed back in all UL subframes.        
A basic problem with ACK/NAK bundling and multiplexing is that a terminal may miss a DL assignment, which may not be indicated in the bundled response. For instance, assume that the terminal was scheduled in two consecutive DL subframes. In the first subframe the terminal misses the scheduling assignment and will not be aware that it was scheduled, while in the second subframe it did successfully receive the data. The terminal will, as a result, transmit an ACK, which the base station will assume holds for both subframes, including data in the first subframe the terminal was not aware of. As a result, data will be lost. The lost data needs to be handled by higher-layer protocols, which typically takes a longer time than hybrid-ARQ retransmissions and is less efficient. In fact, a terminal will not transmit any ACK/NAK in a given UL subframe, only if it missed every DL assignment that was sent during the bundling/multiplexing window associated with the UL subframe.